Use precision that matches the business decision. In most cases, clean arithmetic and correct order of magnitude matter more than carrying unnecessary decimals.

Round when the inputs are already approximate or when a small difference would not change the recommendation. Keep more precision when you are near a threshold, comparing close alternatives, or compounding several steps.

State the approximation before calculating so the interviewer can follow your logic, for example rounding 19.8 to 20 for a quick estimate.

At the end, sanity-check whether the rounded result could materially change the conclusion. If not, move on confidently. For a related reference, see case interview math.

imo the interviewer mainly wants to see that you notice the tradeoff and can choose a sensible next step.